I once had to write a data layer in xml, in-situ with a lifespan for up to hours, as more data was appended to it. An invalid xml document that you couldn't load in many xml apis for 99.9% of its lifespan. I begged and pleaded the lead architect to use an sqlite db for the elements of the data until the transaction was complete and then merely produce the xml file at the end, but no.

It had to survive power outs too.